The following invention relates to magnetic discs and, in particular, a device for increasing the incident energy of ions during the ion beam deposition of a carbon overcoat for a magnetic disc substrate.
Magnetic discs are generally used for storing data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducing heads positioned in close proximity to the recording surfaces of the disks and moved generally radially with respect thereto. Magnetic disks are usually housed in a magnetic disk unit in a stationary state with a magnetic head having a specific load elastically in contact with and pressed against the surface of the disk.
A typical magnetic disc is comprised of a substrate, typically an aluminum (Al)-base alloy, such as an aluminum-magnesium (Alxe2x80x94Mg) alloy, plated with a layer of amorphous nickel-phosphorous (NiP). Deposited on the substrate is a chromium (Cr) underlayer, a cobalt (Co)base alloy magnetic layer, a protective carbon overcoat 13 and a lubricant topcoat. The Cr underlayer, the Co-base alloy magnetic layer and the protective carbon overcoat 13 are typically deposited using sputtering techniques.
The process of depositing a carbon film on a magnetic disc traditionally involved sputtering a carbon target with a mixture of argon and hydrogen or other gases. Recently, an emerging technology called ion beam deposition has been used to deposit carbon film on a magnetic disc.
Referring now to FIG. 1, there is shown an Intevac MDP 250 deposition machine 1 used for ion beam deposition. Attached to deposition machine 1 is a process chamber 9. A turbomolecular pump 3 is disposed in process chamber 9. Inside deposition machine 1 is a carousel (not shown) that includes a disc pedestal on which a disc to be carbon coated is placed. An ion source 7, typically operating on a feed of hydrocarbon and argon gas, is introduced into process chamber 9.
In operation, the disc is positioned in process chamber 9. Ion source 7, which generates an ion beam consisting of positively charged ions of argon and hydrocarbon, is propelled towards the disc by repulsion from a positively charged anode. Concurrently, pump 3 pulls the gas towards the disc resulting in the disc being coated. The ion beam deposition process requires high gas flows (to ion source 7) and low pressures (created by turbomolecular pump 3) in order to establish an incident energy level (the energy of the ions upon arrival at the substrate surface) sufficient for manufacturing throughput. It has been found that an incident energy of approximately 75-100 eV/carbon atom is optimum for the synthesis of the hardest and most dense carbon films. (See J. Robertson, xe2x80x9cAmorphous Carbonxe2x80x9d, Advances in Physics, Vol. 35, No. 4, 1986).
A drawback of the prior art ion beam deposition techniques is that because the incident energy is affected by the operating parameters of the gun as well as the pressure in the deposition chamber, it is often necessary to increase the energy of the ions leaving the ion source to achieve the optimum incident energy. Higher pressures increase the probability of energy exchange occurring between a neutral background gas species (non-ionized argon and hydrocarbon) and the energetic ions. This causes the ion becomes thermalized with a very low energy and the neutral gas acquires approximately the energy the ion had before the collision. Higher pressures also attenuate the energy of the ions as the ions collide with the neutral gas molecules. As a result, the incident energy of the ions is reduced which results in the density of the carbon layer to be reduced as well.
Referring now to FIG. 2, there is shown a chart showing the momentum loss of ions having an original energy of 100 eV, for three different pressures in chamber 9. For example, at 1.5 mTorr pressure, the ions leave ion source 7 with an energy of 60-100 eV. By the time the ions reach the substrate, their energy reduced to approximately 40-60 eV due to charge-exchange with neutral gas atoms and collisions and momentum loss caused by background gas molecules. Because the incident energy of the ions is greatly reduced by these factors, it is necessary to raise the original energy of the ions leaving the source to achieve optimum incident energy. Accordingly, it is desirable to provide an apparatus in which a lower original energy produces the optimum incident energy for carbon coating using disc ion beam deposition.
The present invention is directed to a device for increasing the incident energy of an ion for coating a disc in an ion beam deposition process. The ion beam deposition process is performed in a chamber with the disc to be coated disposed therein. An ion source, having a voltage level, is introduced into the chamber for generating an ion beam for depositing ions on the disc. The present invention includes a bias contact coupled to the disc and a power supply coupled to the bias contact. The power supply applies a voltage level to the bias contact that is less than the voltage level of the ion source thereby creating a negative bias voltage between the disc and the ion source. This negative bias voltage causes the incident energy of the ion to increase. As a result, the optimal incident energy can be achieved.
The invention accordingly comprises the features of construction, combination of elements and arrangement of parts which will be exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.